Trouble with the central locking
If you have the original remote control fitted, the passenger's side door lock does not activate the central locking by default: the central unit has only two triggering inputs, so Citroën had to use the second one either for that door or for the remote control.
However, if you really have a problem with the central locking, remove the internal trim of the door in question. Disconnect the 3-pin white connector of the door lock switch and the 2-pin brown connector of the lock motor, treat them with contact cleaner and reconnect. If this doesn't help, check the rods and nylon fasteners connecting the various parts of the lock system (compare to the other door if necessary).
If the lock obeys the central locking only does not activate it, check the lock switch: disconnect the white connector again. The switch contacts pin 3 of the connector to pin 1 when the door is locked and to pin 2 when it is unlocked (or vice versa, this cannot be seen on the wiring diagram. If in doubt, check on the other door which is correctly functioning). If everything is functional so far, go to the central control unit beneath the center (handbrake) console—secured by two bolts, the first in front of the handbrake, beneath a black plastic cover, the second beneath the cubby at the rear end. You'll have to disconnect the window lifter switches, too. The central locking unit is there, a black box with a brown 9-pin connector. The central contacts from both door lock switches (pin 3 of the white connectors in the doors) go to pin 2, the other two contacts are pin 5 and 7 for the left door, pin 6 and 8 for the right one. Check the same as you did at the door switch. If the contacts are OK, you'll have every reason to suspect the central unit.